A.Daviel
INTERNET-DRAFT Vancouver Webpages
May 1997 (Expires Nov. 1997)
HTTP and HTML Metadata Linking Mechanism
Status of this Memo
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Abstract
This memo describes a method of linking resource metadata
to resource objects using HTTP headers or HTML hyperlinks.
The method uses features which exist in HTTP/1.0 and HTML 2.0,
namely the Link element, to reference metadata of arbitrary
MIME type from resources of arbitrary MIME type.
1. Introduction
Resources of various types may have metadata associated with them.
Where this metadata, and often the resources, are available on
the World-Wide Web, resource discovery and indexing may be
performed by automated agents which traverse hyperlinks between
Web objects. In cases where the resource or the metadata do not
support hyperlinks, links in the HTTP protocol header may be used
in place of links in the body. Examples of such resources include
JPEG images and PDF documents.
This memo describes a method which may be used with existing
servers and software, and is readily available to most Web authors.
It is not intended to take the place of structured metadata
support currently in development for deployment with future
versions of HTTP, but may co-exist in many cases
with such support. It may be used where legacy metadata is defined
in ASCII or binary format but no crosswalk or encapsulation method
has been defined to convert to new structured types.
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1.1 Terminology
A forward metadata link is from a resource to its metadata, and
is indicated by a REL relationship.
A reverse metadata link is from metadata to the described resource,
and is indicated by a REV relationship.
A metadata scheme is a recognized method of describing and
formatting metadata, such as Dublin Core, FGDC geographic
metadata, MCF etc.
If no scheme is specified, the metadata is assumed to follow
common HTML practice (the resource is described in HTML, with
optional Keywords and Description META tags, and may be indexed
using full-text techniques).
2. Application
2.1 HTTP transport
Where a resource is available by HTTP transport, the following
forward link may be used to indicate the URI of corresponding
metadata:
"Link" ":" "" ";" "rel" "=" "meta" *( ";"
"scheme" "=" scheme )
Where metadata is available by HTTP transport, the following
reverse link may be used to indicate the URI of the
corresponding resource:
"Link" ":" "" ";" "rev" "=" "meta" *( ";"
"scheme" "=" scheme )
Examples of usage include:
Link: ;
rev="META"; scheme="DC"
Link: ; rel="META"
Link: ;
rel="META"; scheme="FGDC"
2.2 HTML encoding
Where a resource is available in HTML, a forward link in the
document head may be used to indicate the URI of corresponding
metadata:
""
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A forward anchor construct may be used in the document body
where a visible link is required:
""
label ""
Where metadata is available in HTML, a reverse link may
be used to indicate the URI of the corresponding resource:
""
A reverse anchor construct may be used in the document body
where a visible link is required:
""
label ""
Examples of usage include:
Barkley Sound
2.3 Resource Indexing
If a reverse metadata link is present in a document head,
document body or HTTP header, the URI indexed by a discovery
agent should be that of the resource, not that of the
metadata.
2.3.1 Link Priority
Multiple metadata links using the same scheme are deprecated.
Where multiple metadata links using the same scheme are present,
only one link should be used by a discovery agent. HTTP headers
should be given a higher priority than HTML link and
anchor elements.
If a metadata link is present in the HTTP header of an object, the
body may be ignored, so that an HTTP HEAD request may be used.
Therefore if any metadata link is present in the HTTP header, all
links using alternative schemes must also be present.
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2.3.2 Metadata Spoofing
In order to minimize the possibility of metadata spoofing
( one organization supplying misleading metadata for a resource
belonging to another organization), the forward relationship
should have priority over the reverse relationship.
Therefore, if a document defining a reverse relationship is
traversed, a request should immediately be made for the
resource and any forward link traversed in turn to discover
definitive metadata for the resource.
If any metadata exists for a resource defined by a
forward relationship, metadata defined only by a reverse
relationship should be considered invalid for all schemes.
For example, Dublin Core metadata defined with both forward and
reverse links invalidates MCF metadata defined with only a
reverse link. If the author has the ability to add a forward
link for one scheme, they are assumed to have that ability for
all schemes.
If no forward metadata links exist, multiple reverse metadata
links may optionally be resolved in favour of the link most
closely matching the host and path of the resource URI.
These requirements reflect the fact that in many situations
an author currently has control only over HTML documents,
and has limited access to HTTP server features.
2.3.3 Offline Resources
Where a resource is not available online, a null URI may be used
in the reverse link to so indicate. Resources such as paper maps
or audio CDs may thus be indexed by means of online metadata.
The metadata scheme used may indicate the physical availability
and format of the resource.
Example:
(Such a provision may seem to be pointless, but it would
enable a search for a book by ISBN where it is clear
to the agent that the object itself is offline, and
conversely allow a search for online objects only if
desired.)
3. Example
A short example is available at http://vancouver-webpages.com/ml/
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4. References
[RFC2068] HTTP/1.1
[RFC1945] HTTP/1.0
draft-musella-html-metatag-03.txt, Davide Musella
http://www.w3.org/pub/WWW/MarkUp/html-spec/html-spec_5.html#SEC5.2.4
http://www.w3.org/pub/WWW/TR/REC-html32.html
5. Author's Address
A.Daviel
Vancouver Webpages Box 357
185-9040 Blundell Rd
Richmond BC
V6Y 1K3
Tel. (604)-377-4796
Fax. (604)-270-8285
mailto:andrew@vancouver-webpages.com